The 22-year-old, who entered the game as the Broncos' leading rusher with 176 yards through three games, was unable to put full weight on his injured leg and taken to the locker room on a cart.
Williams was seen on crutches after the game and while Hackett would not elaborate on the extent of the injury amid fears of a lengthy stint on the sidelines, he confirmed he will need scans on Monday.
"We're still evaluating," Hackett said. "We want to be sure we get MRIs and get a feel there."
Broncos quarterback Russell Wilson added: "Obviously seeing Javonte go down, that hurt. Hopefully he'll be OK.''
Williams ran for 28 yards on 10 carries in Sunday's game. He has been the Broncos' second-leading receiver with 16 catches for 76 yards this season.
The Raiders' win marked Josh McDaniels' first as head coach of the franchise, coming against the team he led for almost two years before being sacked in December 2010 after losing 12 of his last 17 games with Denver.
"They gave me a great opportunity at a young age and, if anything, I look at it like it was a blessing because I learned so much from it," McDaniels told reporters.
"That made me better and I've been forever grateful to [owner] Mr [Pat] Bowlen for what he gave me an opportunity to do. Unfortunately, I didn't do well enough with it, but hopefully I've learned from it."